1. Field of the Invention
The present invention relates to a printer, a copier, a facsimile, or the like, and to a halftoning device or a halftoning method that enables grayscale of 256 levels (8 bits) or more by combining a plurality of pixels. In the present invention, a raster device that forms an image in the form of a rectangular array of the pixels, such as a printer, a copier, a facsimile, or the like, is typically called a printer.
2. Description of the Related Art
In order to cause the printer to represent 256 levels (8 bits) of gradation called continuous tone, there must be performed image conversion which effects halftone representation as changes in areas where a plurality of pixels are ON state and is called halftoning.
Common halftoning methods include an ordered dither method for determining the ON/OFF state of a pixel by comparing a value in a previously-prepared array of threshold values with an input pixel value; and an error diffusion method which re-distributes an error, which arises with reference to an input gradation value as a result of ON/OFF state of the pixel, over peripheral pixel values.
Further, ordered dither methods include clustered-dot halftoning for forming periodic clustered-dots from a plurality of adjacent pixels and effecting gradation expression by changing in the area of the clustered-dots, and a diffused halftoning (Bayer) method for homogeneously distributing ON dots in given areas in a diffused manner.
In a laser type printer, where reproduced pixels are likely to become unstable, among the halftoning methods a clustered-dot halftone method is preferably used, because the clustered-dot halftone method enables to lower an influence of fluctuations of one pixel to a gradation characteristic and is superior in terms of gradation representation stability.
Meanwhile, a trade-off exists between the density of clustered-dots and a gradation characteristic during clustered-dot halftoning. A monochrome binary printer of 600 dpi encounters a problem in achieving compatibility between a gradation characteristic and a resolution. For example, in the printer of 600 dpi, when each of clustered-dots is formed from 3-by-3 pixels, a resolution of 200 dpi=600/3 is achieved. The acquired number of steps of gradation, including a white background, is merely 3×3+1=10. In order to cause a printer, which provides two steps of gradation for one pixel, to provide 256 steps of gradation, one clustered-dot is formed from 16×16. The density of clustered-dots comes to 600/16=37.5 dpi. Thus, very coarse clustered-dots are acquired.
In the case of a printer of laser beam type or LED array type, a method for subdividing one pixel of the printer by PWM (Pulse Width Modulation) is available in relation to the above problems. Further, there is used a method for generating multi-gradation from one dot by a PAM (Pulse Amplitude Modulation) method for modulating exposure intensity of one dot.
However, even in the case of these methods, a pulse interval for one pixel becomes shorter in response to a recent speedup of the printer. For this reason, there arises a problem of requiring multiple laser beams in order to sub-divide a pulse because of the response speed of pulse generation or limitations on development following capability, or a problem of difficulty in achieving compatibility between multilevels of intensity variation and cost reduction, such as an increase in development cost because of complication of an LED.
In order to solve these problems, U.S. Pat. No. 6,191,868 and JP-A-2001-94782 describe a method for effecting distribution while achieving a compatibility between clustered-dot halftoning with multiple center dots that grow through cyclic dispersion, and PWM. JP-A-2002-185787 describes, as a method for storing a threshold value array, a method for computing, from a basic threshold value pattern, an interval between threshold values assigned essentially uniformly to the number of output gradation steps and generating an array of threshold values.